1. Technical Field
The present invention relates to an electrostatic actuator used for an inkjet head driven by an electrostatic force, a droplet discharging head, a method of manufacturing thereof and a droplet-discharging device.
2. Related Art
As a droplet discharging head, an inkjet head driven by an electrostatic force is known and facilitated for an inkjet recording device. An inkjet head driven by an electrostatic force generally comprises an electrostatic actuator including an individual electrode (a fixed electrode) formed on a glass substrate and an oscillating plate (a movable plate) made of silicon and placed at a position opposing the individual electrode via a predetermined gap. The ink jet head further comprises a nozzle substrate in which a plurality of nozzle holes are formed for discharging ink droplets, a discharging chamber communicated with the nozzle holes by being coupled to the nozzle substrate, a cavity substrate in which an ink flow channel such as a reservoir is formed, applying a pressure to a discharging chamber by generating an electrostatic force in the electrostatic actuator and discharging ink droplets from selected nozzle holes.
The conventional electrostatic actuator includes an insulating film on an opposing surface of the oscillating plate or the individual electrode in order to secure driving stability and endurance by avoiding dielectric break down or electric short cut of an insulating film in the actuator. Such insulating film is generally made of thermally oxidized silicon. The reason for using thermally oxidized silicon is simplicity in a manufacturing process and superior insulating characteristics. Further, JP-A-2000-318155 discloses an electrostatic actuator in which an oxidized silicon film and nitride film are used. JP-A-8-118626 and JP-A-2003-80708 further disclose an electrostatic actuator in which an insulating film is formed on both the oscillating plate and the individual plate. JP-A-2002-46282 further discloses an electrostatic actuator in which a double layered (a high volume resistance layer and a low volume resistance layer) protection film is formed only on a surface of the individual electrode. JP-A-2006-271183 further discloses an electrostatic actuator in which an insulating film made of a dielectric material, of which relative permittivity is higher than that of oxide silicon such as a high-k material (a gate insulating film having high relative permittivity) is formed to improve a generation pressure of the actuator.
The above conventional technologies, however, have a problem of limitation in which thermal silicon oxide film as an insulating film for an electrode of an electrostatic actuator is applied to only a silicon substrate. Hence, the thermal silicon oxide film is formed only on a side of oscillation plate, which is a movable electrode. On the other hand, as shown in JP-A-8-118626, when a SiO2 film is used, many carbon additives are mixed in the film during a manufacturing process such as a CVD method. The mixture results in instability of the film such as wear of the film due to repeated contact with an individual electrode through an endurance examination.
JP-A-8-118626 shows that a thermal silicon oxide film is formed on the side of the oscillating plate and an oxide silicon film (a sputtered film) is formed on the individual electrode by sputtering. But, the dielectric breakdown voltage of the sputtered film is so low that the thickness of it should be thicker or another film having a high dielectric breakdown voltage such as thermal oxide film should be formed on the side of the oscillating plate in order to avoid insulation break down of an electrostatic actuator.
Further, JP-A-2003-80708 shows that both the oscillating plate and the individual electrode are made of silicon, thermal oxide films are formed as an insulation film on them and an insulating film is not formed on a contact surface to the silicon substrate. A silicon substrate, however, is more expensive than a glass substrate, resulting in high manufacturing cost.
JP-A-2002-46282 shows a double-layered film for a protecting electrode composed of high volume resistance layer and low volume resistance layer only on the individual electrode and the oscillating plate is made of a metal such as molybdenum, tungsten, and nickel. Such insulation structure, however, results in complex structures in an electrostatic actuator, complex manufacturing processes, becoming high cost.
JP-A-2006-271183 shows that a insulating material of which permitivity is higher than that of oxide silicon is used as a insulating film of an actuator, which is shown in the formula (2) described later, enhancing a generation pressure of an actuator. Driving an actuator requires application of a voltage to a space between electrodes. However, such applied voltage is limited to be low if the dielectric breakdown voltage of an insulation film on an electrode is low. Such limitation results in difficulty in improving the above generation pressure of an actuator in case when the dielectric breakdown voltage of a high-K material is lower than that of oxide silicon since the applied voltage V should be small based on the formula (2).
Further, the above all patent documents do not disclose that an insulation film is formed by an atomic layer deposition (ALD) method.
On the other hand, in recent years, the demand of high density with high resolution and high driving speed in an electrostatic driving ink jet head provided with a electrostatic actuator is sharply increased, making an electrostatic actuator further miniaturized. In order to response such demand, a further improved insulating structure of an electrostatic actuator is a key factor, enhancing the generation pressure of an electrostatic actuator with a low cost, stability of driving it and driving endurance.